1. Field of the Invention
The present invention relates to electrographic imaging and more particularly to improved apparatus and techniques for use in mist development in electrographic imaging.
2. Brief Description of the Prior Art
In the early stages in evolution of electrographic copying and printing techniques, it was recognized that certain advantages pertain to development of latent electrographic images with small, airborne marking particles, e.g., in "liquid mist" or "powder cloud" form (hereinafter collectively referred to as "mists"). A primary advantage envisioned for mist development was the achievement of higher resolution development and thus better quality in fine line detail of graphic reproductions, or in half-tone and continuous tone reproductions.
Early systems seeking to obtain such advantages introduced a mist of electrostatically-charged marking particles between an image member, bearing an electrostatic image to be developed, and a development electrode which was spaced closely to the image bearing surface and connected to a source of potential. The electrical field thus created between the development electrode and the electrostatic image caused movement of the charged particles toward the image. However, commercialization of these early mist systems was impeded by the tendency of the charged marking particles to mutually repel one another and to attach to the supply conduit. These effects presented great difficulty in production and transport of a uniformly charged and concentrated mist. As a result, mist systems were not actively developed; and, at present, the most common commercial technique for developing latent electrostatic images on dielectric supports is by contacting such images with triboelectrically-charged marking particles, in mixture with a particulate or liquid carrier medium.
Recently, however, electrographic imaging and development systems have been disclosed which utilize a mist of substantially-neutrally-charged ink particles. In one such system the neutral-charge ink mist is introduced between a copy sheet (during its movement over an electrode) and an apertured or grid array which is electrically-biased according to the image pattern to be reproduced. The pattern-biased array modulates the passage of an ion stream located on the opposite side of the array from the copy sheet and directed toward the receiver. The ions that pass through the modulator charge ink particles in their path, and the charged particles are attracted toward, and deposit on, the receiver (see, e.g., U.S. Pat. No. 3,779,166).
Another recently developed approach using neutrally-charged ink mist is disclosed in copending U.S. application Ser. No. 805,216 entitled "Improved Electrographic Imaging Apparatus and Method" and filed June 9, 1977 in the name of Clark N. Kurtz. In that system, a mist of neutrally-charged ink particles is introduced between an image element having an electrical pattern to be developed and a predeterminedly-electrically-biased, ion control member. Upon directing a stream of ions through the control member and toward the mist, an imagewise-varying electrical field across the mist, viz., between the image element and the control member, controls charging of the mist particles and thus deposition of the particles on the image element. Such deposition is precisely in accordance with the outline of electrical pattern on the image element and in proportion to the relative magnitude of the pattern portions.
Both of the neutrally-charged ink mist systems described above have been demonstrated to provide useful results. However, it would be highly desirable, in some applications, to have images of density higher than has thus far been attainable by those systems. That is, both of the ink mist deposition systems described above have thus far been limited, in density attainable, by the atomization and covering characteristics of the ink solutions and particle dispersions. For example, an increase in concentration of colorant in the ink solvent has been found to increase the ink viscosity and thus reduce the atomization capabilities for the ink.